1. Field of the Invention
The invention concerns integrated circuits and, more particularly, it concerns the way to make a constant current source, in these circuits, that is stable as a function of the temperature and the supply voltage of the integrated circuit.
2. Description of the Prior Art
There are known current sources made with a field-effect transistor and a reference voltage source that biases the gate of this transistor. The reference source voltage may be of the so-called "bandgap" type. The term "bandgap" refers to the energy interval between the valence bands and the conduction bands of a semiconductor. Sources of this type use the known relationship of dependency between this interval and the temperature to achieve compensations that make the reference voltage as stable as possible as a function of the temperature.
A voltage source of bandgap type generally has two diodes through which there flow different currents (or the same currents, but in this case the diodes are obligatorily ones with different junction surfaces) and a looped differential amplifier amplifying the voltage difference at the terminals and supplying the diodes with current.
A reference voltage source of this type is shown in FIG. 1. We shall return further below to the detailed description of this circuit.
It is of course possible to make a current source out of this voltage source, but the stability in temperature is lost during the voltage/current conversion.
There are also known references sources called "Wilson mirror" sources. A source of this kind is shown in FIG. 2. It is based on the mutual compensations of variations in characteristics of several transistors which copy one another's currents mutually.
To put it schematically, a Wilson mirror source has two parallel branches with two transistors each, and the transistors are mounted so that each branch copies the current of the other one, two transistors (each belonging to a different branch) being different in size or in threshold voltage.
Here again, although the stability obtained is often considered to be satisfactory, it is not perfect.
There are yet other reference voltage sources which do not have to be gone into in detail herein.